powerpc/boot/install.sh: Fix shellcheck warnings

Fix shellcheck warning such as
"Double quote to prevent globbing and word splitting." and
Use $(...) notation instead of legacy backticks `...`.

Tested-by: Venkat Rao Bagalkote <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/boot/install.sh

···
 set -e
 
 # this should work for both the pSeries zImage and the iSeries vmlinux.sm
-image_name=`basename $2`
+image_name=$(basename "$2")
 
 
 echo "Warning: '${INSTALLKERNEL}' command not available... Copying" \
      "directly to $4/$image_name-$1" >&2
 
-if [ -f $4/$image_name-$1 ]; then
-	mv $4/$image_name-$1 $4/$image_name-$1.old
+if [ -f "$4"/"$image_name"-"$1" ]; then
+	mv "$4"/"$image_name"-"$1" "$4"/"$image_name"-"$1".old
 fi
 
-if [ -f $4/System.map-$1 ]; then
-	mv $4/System.map-$1 $4/System-$1.old
+if [ -f "$4"/System.map-"$1" ]; then
+	mv "$4"/System.map-"$1" "$4"/System-"$1".old
 fi
 
-cat $2 > $4/$image_name-$1
-cp $3 $4/System.map-$1
+cat "$2" > "$4"/"$image_name"-"$1"
+cp "$3" "$4"/System.map-"$1"